Why Donald Trump's days may be numbered

The Republicans are living their Herb Stein moment. Stein, an eminent and witty US economist who served under presidents Nixon and Ford, was most famous for the 'Stein doctrine', that "if something can't go on forever, it won't". Something, say, like the marriage made in hell between the Republican party and Donald Trump.

American presidential elections drag on for so long for one very good reason: they give voters a chance to get a feel for a candidate, especially if he hasn't been around politics very long. And boy, has this rationale worked in the case of Trump.

Before he entered the race, he was merely the flashy, ever-bragging entrepreneur, with a knack for TV. Now we know Trump the politician. He's shown himself to be incapable of self-discipline for more than five minutes. His relationship with the truth is next to non-existent.

He's got an incredibly thin skin. He never lets an insult go unreturned. He lacks uttterly that vital political skill of sometimes turning the other cheek.

And, even though he's hopelessly ignorant on policy matters, he seems unwilling to listen to advisers, convinced that he himself is the source of ultimate wisdom on all matters.

This last week has capped the lot. He was crudely and callously dismissive of a Muslim-American couple who had lost a son in Iraq and refused to apologise.

He declined to endorse the party's most senior elected figure, Speaker Paul Ryan. He claimed that Russian forces had not entered Ukraine. He was caught out in a couple of other juicy lies, and even managed to pick a fight with a baby who was crying at one of his rallies.

That alone would be bad enough. But what drives the Republican hierarchy to apoplexy is that 2016 was, and maybe still is, an election ripe for the taking.

This should have been a bumper week for Trump - a legalistic Clinton quasi-lie about how the FBI director had said she was telling the truth over her State Department emails, that would have been grist for the 'Crooked Hillary' mill, and lousy GDP figures that should have dented her economic claims.

All was lost though in the furore over his missteps. Trump's establishment-bashing goes down pretty well among disillusioned voters, but not his bullying and insulting of ordinary people like the Khan family.

If he'd thrown away his phone and gone to a desert island for a fortnight after the Republican convention, he'd probably be ahead right now. Instead, a poll yesterday by Fox News, broadly sympathetic to his cause, puts Clinton in the lead by ten points, one of her biggest yet.

Yes, there are solid reasons why other elected Republicans are holding their noses and sticking for now with Trump, at whatever price for their future reputation.

One is their overarching loathing of Hillary Clinton, the one thing that does unite the party. Another is simple mathematics.

They need the votes of Trump's sizeable and unshakeably loyal following (who poured $80m into his coffers last month) to get re-elected in November.

But as Stein said, this can't go on forever. If Trump keeps piling on the policy blunders and own goals, if the unrest in his own campaign continues to grow - and, most important, if he continues to sink in the polls, then something has to give.

Trump will have to then change his ways, stays on message and takes advice from others. But that's tough for anyone who's 70 and, as a year of campaigning has shown, unlikely.

Alternatively, Republicans are convinced the White House is lost, and conclude that the best hope of saving the party's majorities in Congress, is to repudiate Trump. The former could happen. The latter seems more likely.